This invention relates generally to a combustor, and, more specifically, to a dry-low nitrogen oxide (NOx) (DLN) combustion system for a gas turbine engine.
Combustion systems of at least some known gas turbine engines combust synthesis gas, or syngas, to create exhaust gases that drive a gas turbine. However, some known syngases have a low heating value as compared to other fuels, such as, natural gas, and, as such, may also have a low Modified Wobbe Index (MWI) as compared to other fuels. Additionally, some known syngases have a significant hydrogen content, based on molar fuel fractions, that can result in a highly reactive fuel stream with a very small characteristic chemical time. Due to this combination of low MWI and high fuel reactivity, conventional premixed DLN combustion systems can experience flashback when combusting syngas. “Flashback” refers to a condition that may occur when the aerodynamics of fuel introduction and mixing are overcome by the rapid chemistry of the combustion process thus allowing the reaction to stabilize within the premixing device. It is well established that the characteristic chemical time of the fuel can be used to correlate flashback, and that, the longer the characteristic chemical time, the slower the reaction and hence the lower proclivity of the fuel to induce a flashback event. Over time, occurrences of flashback may be damaging to hardware within the combustor. To reduce flashback occurrences within some known dry-low NOx combustion systems, narrow fuel specifications for both hydrogen content and MWI are required for normal operation.
To dispense with flashback concerns, some known combustion systems that combust syngas are based on diffusion combustors that do not premix fuel with air, and are not susceptible to flashback. Such systems inject a diluent to reduce NOx emissions by suppressing the peak temperatures of the reaction. However, the proximity of the nitrogen supply to the combustion system and the additional compression the nitrogen may require before injection may add complexity and/or cost to the combustion systems, as compared to systems that do not include nitrogen injection into the combustor.
Another known system introduces a nitrogen-water vapor mixture as the diluent, and yet another of the known systems injects a fuel-water vapor mixture into the combustor to control NOx formation, and still another uses carbon dioxide. Ultimately, water availability and water quality may adversely affect such systems, and, as such, combustors using steam injection may require costly and complex steam systems to avoid the adverse effects of the water.